Forming single crystal silicon by various methods is known1. In general, to create silicon in a single-crystal state, high-purity silicon must be melted and reformed or solidified very slowly in contact with a single crystal “seed.” The silicon adapts to the pattern of the single-crystal seed as it cools and gradually solidifies, for example, into a rod or boule of single-crystal silicon. 1http://www.eere.energy.gov/solar/silicon.html; see also, http://www.tf.uni-kiel.de/matwis/amat/semi_en/kap—3/backbone/r3—2—2.html
Once typical single-crystal rods are produced, by either the Cz or FZ method described herein, they must be sliced or sawn to form thin wafers. This sawing process, however, wastes as much as 20% of the valuable silicon as dust, known as “kerf.” Further, the Cz and FZ processes are complex and expensive.
In the Czochralski process, a seed crystal is dipped into a crucible of molten silicon and withdrawn slowly. As it is withdrawn, a cylindrical single crystal forms as the silicon crystallizes on the seed. However, crystals may be contaminated by the crucible used in growing Czochralski crystals.
The float-zone process produces purer crystals than the Czochralski method, since a crucible is not used. In the float-zone process, a silicon rod is set upon a seed crystal, and lowered through an electromagnetic coil. The coil's magnetic field induces an electric field in the rod, heating and melting the interface between the rod and the seed. Single-crystal silicon forms at the interface, growing up the cylinder as the coils are slowly raised.
Another group of crystal-producing processes are referred to as “ribbon growth” processed. These single crystals may cost less than other processes, since they form the silicon directly into thin, usable wafers of single-crystal silicon. These methods involve forming thin crystaline sheets directly, thus avoiding the slicing step required of cylindrical rods.
One “ribbon growth” technique is known as edge-defined film-fed growth. In this process, two crystal seeds are grown and a sheet of material is captured between them as they are pulled from a source of molten silicon. A frame holds a thin sheet of material when drawn from a melt. This technique does not waste much material. However, a key limitation is that the purity of the material formed from conventional silicon ribbon growth processed is not as high as silicon formed by the Cz and FZ methods. This is not acceptable for many applications. Further, it reduces the efficiency in applications where it is acceptable, such as certain photovoltaic cell processes.
One example of a process for forming silicon ribbon is disclosed in U.S. Pat. No. 4193,974: Process for producing refined metallurgical silicon ribbon.
Another well known process of the fabrication of low loss optical fibers for wide bandwidth communications, which have become ubiquitous. Their manufacture relies on having ultra-pure glass pre-forms (in the form of cylindrical rods) made of about 94% SiO2 and 6% GeO2. These are usually made from oxidizing SiCl4 and GeO2 which are know to have ultra-high purity, with impurity levels in the sub 1 part per billion. These pure glass pre-forms are then heated to an appropriate state of softness to facilitate drawing fibers having diameters in the range of 1-1000 microns and lengths of tens of kilometers.
Conventional attempts to produce pure optical fibers (e.g., sub 1 part per billion impurities) follow the following general steps as illustrated in FIGS. 1A-1B.
Step 1: Providing silica or quartz powder with purity in the range of 97%-99.9%. The heating the powder to a high temperature near or above the melting temperature of˜1870° C. in the presence of a reducing agent like coke or other carbon. The SiO2 reduces to Si and CO2. The silicon produced in this manner is called Metallurgical Grade Silicon or MGS.
Step 2: The MGS is reacted with HCl and Cl2 to respectively produce SiCl4 and SiHCl2 gases.
Step 3: The SiCl4 and SiHCl2 thus produced are purified by means of fractional distillation process that has been shown to reliably produce purities of sub 1 part per billion. This is step is costly and requires precautions constrained by safety and environmentally considerations due to hazardous toxic waste materials.
Step 4: Providing GeO2 powder with purity in the range of 97%-99.9%. The heating the powder to a high temperature near or above the melting temperature in the presence of a reducing agent like coke or other carbon. The GeO2 reduces to Ge and CO2. The germanium produced in this manner is called Metallurgical Grade germanium or MGG.
Step 5: The MGG is reacted with HCL and Cl2 to respectively produce GeC4 and GeHCl2 gases.
Step 6: The GeCl4 and GeHCl2 thus produced are purified by means of fractional distillation process that has been shown to reliably produce purities of sub 1 part per billion. This is step is costly and requires precautions constrained by safety and environmentally considerations due to hazardous toxic waste materials.
Step 7: The ultra-pure SiCl4 and GeCl4 are oxidized to produce ultra-high purity (sub 1 part per billion) SiO2 GeCl2 soot that is collected on a cylindrical substrate such quartz tubes and becomes the perform from which the glass fibers are drawn. This step also adds to the cost and also is constrained by environmental and safety constraints.
Step 8: The glass soot on the substrate is sintered to produced dense pre-form.
Step 9: The pre-form in an optical fiber manufacturing facility is heated to temperatures about 2000° C. and so that it can be drawn into tens of km long fibers with diameters ranging from 1 micron to 100 microns.
The conventional method described generally above has many steps which are expensive requiring heavy capital investment. This includes costly strategies for safety, environmental protection and disposal of hazardous material. Normally, these factories are designed to produce high volumes to justify the heavy capital investment and produce a final product that is priced advantageously for market acceptance. What also adds to the cost of prior method are strategies for recycling the germanium gases, the helium, and all the Cl based materials and gases released during the various processes.
Another key limitation of conventional methods is the discontinuity of manufacturing steps. This means that not all steps are carried our “under one roof” by one manufacture. The process involves different entities with different expertise. This adds cost due to the storage, transportation and sometimes duplicate infrastructure in different installations to cope with the safety and environmental hazardous materials.
It is highly desirable to seek an alternative method of manufacturing sub 1 part per billion SiO2 that has the following characteristics: 1) Reduces cost by eliminating certain steps. 2) Eliminates the steps that are generally unsafe and are unfriendly to the environment. 3) Scalable from small low volume sized to large sizes. 4) The final product has multi uses instead of merely used for making optical fibers. This leverages the infrastructure to produced more profit. 5) Continuous “under-one-roof” manufacturing.